Adding base

Makefile

@@ -0,0 +1,13 @@
+CC=clang
+CFLAGS=-pedantic -Wall -Wextra -std=c99 -D_GNU_SOURCE -O3
+
+all: shorten
+
+shorten: shorten.o
+	$(CC) $(CFLAGS) -o $@ $^
+
+%.o: %.c
+	$(CC) $(CFLAGS) -c $< -o $@
+
+clean:
+	rm *.o

shorten.c

@@ -0,0 +1,128 @@
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/param.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#define BUF_CAP 1024
+
+int main(int argc, char **argv) {
+  if (argc != 3) {
+    printf("%s: Invalid number of arguments.\n", argv[0]);
+    printf("Usage: %s linecount filename\n", argv[0]);
+    printf("to remove linecount lines from the end of the file\n");
+  }
+
+  int number = atoi(argv[1]);
+  char *file = argv[2]; // No copy
+  int count = 0;
+
+  if (number <= 0) {
+    printf("Bad number of line\n");
+    exit(1);
+  }
+
+  struct stat file_stat;
+  if (stat(file, &file_stat) < 0) {
+    printf("Stat failed: %d\n", errno);
+    return 2;
+  }
+  int file_size = file_stat.st_size;
+  if (file_size == 0) {
+    printf("No change: file does not end with a newline\n");
+    return 1;
+  }
+
+  int fd = open(file, O_RDWR | O_APPEND);
+
+  if (lseek(fd, -1, SEEK_END) < 0) {
+    printf("Lseek1 failed: %d\n", errno);
+    close(fd);
+    return 2;
+  }
+  char c;
+  if (read(fd, &c, 1) < 0) {
+    printf("Read1 failed: %d\n", errno);
+    close(fd);
+    return 2;
+  }
+
+  if (c != '\n') {
+    printf("No change: file does not end with a newline\n");
+    close(fd);
+    return 1;
+  }
+
+  while (1) {
+    // We can't seek back before the beginning of the file
+    int buf_size = MIN(BUF_CAP, file_size);
+    char buf[buf_size];
+
+    // Go back in the file to read the length of the buffer
+    if (lseek(fd, -buf_size, SEEK_CUR) < 0) {
+      printf("Lseek2 failed: %d\n", errno);
+      close(fd);
+      return 2;
+    }
+
+    // ~lseek(fd, buf_size, os.SEEK_CUR)
+    int buf_len = read(fd, buf, buf_size);
+    if (buf_len < 0) {
+      printf("Read2 failed: %d\n", errno);
+      close(fd);
+      return 2;
+    }
+
+    char *char_pos = memrchr(buf, '\n', buf_len);
+    if (char_pos != NULL) {
+      count++;
+      // Go to position found ; a read would read a '\n'
+      if (lseek(fd, (ssize_t)(char_pos - buf) - buf_len, SEEK_CUR) < 0) {
+        printf("Lseek3 failed: %d\n", errno);
+        close(fd);
+        return 2;
+      }
+      // "resize" the file to look for next '\n'  from this point
+      file_size -= buf_len - (ssize_t)(char_pos - buf);
+    } else {
+      // Go back to previous position
+      if (lseek(fd, -buf_len, SEEK_CUR) < 0) {
+        printf("Lseek4 failed: %d\n", errno);
+        close(fd);
+        return 2;
+      }
+      file_size -= buf_len;
+    }
+
+    if (file_size < 0)
+      break;
+
+    if (count == number + 1) {
+      file_size++; // Keep the last '\n'
+      if (ftruncate(fd, file_size) < 0) {
+        printf("Ftruncate failed: %d\n", errno);
+        close(fd);
+        return 2;
+      }
+      close(fd);
+      return 0;
+    }
+
+    // Beginning of file?
+    if (lseek(fd, 0, SEEK_CUR) == 0) {
+      printf("No change: file does not end with a newline\n");
+      return 1;
+    }
+  }
+
+  close(fd);
+
+  if (count < number + 1) {
+    printf("No change: requested removal would leave empty file\n");
+    return 3;
+  }
+}